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CHAPTER 1 – EARTH'S DYNAMIC SURFACE
A braided, high gradient stream roars down a canyon in the Tien Shan Mountains,
Kyrgyzstan. The large levees along the channel that are studded with huge boulders
indicate that the channel was impacted by debris flows. Photo Marli Bryant Miller.
GEOMORPH0000001330.
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INTRODUCTION
GEOSPHERE
Tectonics
Lithology and Structure
HYDROSPHERE
Climate and Climate Zones
Hydrologic Cycle
BIOSPHERE
Geographical Distribution of Ecosystems
Humans
LANDSCAPES
Process and Form
Spatial Scales
Temporal Scales
UNIFYING CONCEPTS
Conservation of Matter and Energy
Material Routing
Force Balances and Thresholds
Equilibrium and Steady-State
Recurrence Intervals and Magnitude/Frequency Relationships
APPLICATIONS
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INTRODUCTION
Geomorphology is the study of the processes shaping Earth's surface and the
landforms that they produce. The word itself was introduced into the English language in
the late 19th century, made from the combination of three Greek words: geo (earth),
morphos (form) and ology (the study of). Geomorphologists learn to read landscapes in
order to understand the processes shaping Earth's surface, decipher the physiographic
history of a place, and recognize (and potentially mitigate or manage) the environmental
impacts of ancient and modern societies. Whether one is on the way to work or exploring
remote corners of the globe, geomorphology offers insight into how water and sediment
move, rocks break down to create soil, and the tectonic uplift that raises mountains is
locked in a geologic duel with the erosional processes that tear them down.
Geomorphology provides the uninitiated a new way to see the world because landscapes
tell stories that can be read using basic geological principles.
Until relatively recently, geomorphology was a descriptive science. Classification
of landforms and topographic features dominated geomorphic thought. In the latter half
of the 20th century, geomorphology underwent a dramatic transformation in the
understanding of the processes driving the development and evolution of topography. As
the plate tectonics revolution illuminated the mystery of mountain building and solved
the problem of continental drift, radiometric dating techniques revealed time scales, and
developments in electronics, sensors, and computational techniques allowed quantitative
characterization of geomorphological processes as well as the connection of process to
form. Today, geomorphology is an integrative science that brings a wide range of
methods and techniques to bear on understanding the processes that shape Earth's
dynamic surface.
Geomorphology is an inherently synthetic discipline that draws on geology,
physics, chemistry, and ecology to understand landscape-forming processes and decipher
how weathering, erosion, and deposition sculpt the land. It is a science with innate
intellectual and aesthetic appeal. It also has fundamental societal relevance because we
live on Earth's surface, and the dynamic processes that shape topography influence
human societies by controlling things like where and when landslides and floods occur,
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the geography of erosional uplands and depositional lowlands, and the kinds of soils and
plants one finds in different parts of the world (Photo 1.1). In practice, geomorphology
is interdisciplinary because topography reflects the interaction of driving forces that
elevate rocks above sea level, the erosional forces that wear rocks down, and the factors
that determine resistance of the land surface to erosion — including how weathering
progressively weakens slope-forming materials and how vegetation reinforces and
stabilizes them.
Most people think of the land surface as stable and unchanging because we don’t
usually notice subtle, ongoing changes in our daily experience. But geomorphologists
see landscapes as constantly changing and evolving, usually slowly but sometimes
catastrophically shaping and re-modeling our world. Knowing what to look for —
knowing how to see — is the key to understanding landscapes. And seeing landscapes as
dynamic systems is central to understanding how natural forces shape Earth’s surface,
influence land use, and respond to human actions.
Topography reflects an ongoing competition between the processes that build up
and wear down Earth's surface. Tectonics and volcanism drive the endogenic (internal,
or generated from within) processes that raise mountains and elevate topography above
sea level. In these ways, Earth's internal dynamics provide the raw material for erosional
processes to work on in shaping landscapes and landforms. The vast array of erosional
processes that influence Earth's surface arise from exogenic (external, or generated from
outside) processes imposed on landscapes through the action of wind, water, and ice.
Endogenic processes are an example of a closed system driven by Earth's internal heat,
which drives plate tectonics through deep convection of the planet's mantle. Exogenic
processes are driven by the Sun's energy and the temperature gradients between the poles
and equator, and are examples of open systems in which external inputs of energy drive
the evaporation that introduces moisture the atmosphere and the weather patterns that
deliver it to the continents. The potential energy of water in the atmosphere was obtained
through evaporation and atmospheric processes that transport water against gravity. The
global competition between uplift and erosion defines a never ending and ever shifting
driver shaping the world we know and live on.
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In framing how to study geomorphology, we start with three broad influences on
geomorphological processes. Specifically, we focus on the geosphere that includes the
rocks comprising Earth's crust and the global tectonic system that elevates the rocks that
gets sculpted into topography, the hydrosphere that encompasses the oceans,
atmosphere, and the surface and subsurface waters that erode, transport and deposit
sediment, and the biosphere that consists of ecological systems that shape and are in turn
shaped by landscape dynamics. Global patterns within and among these three basic
systems set the broad regional templates within which geomorphological processes shape
landforms and landscapes evolve.
GEOSPHERE
Deep earth and planetary-scale processes drive the motion of Earth’s tectonic
plates and control the locations of mountains, plains, plateaus, and ocean basins. The
most obvious feature of global topography, the contrast between average land surface
elevations on the continents versus seafloor depths in ocean basins, reflects long-term
recycling of Earth’s crust over geologic time to produce continents rich in elements
lighter than those that compose oceanic crust. The resulting differences in the density of
continental and oceanic crust give global topography two dominant elevations (Figure
1.1). The land surface of the lighter continents stands an average of almost 1 km above
sea level. The denser crust of the deep ocean floors sits about 4 km below sea level. The
spectacular mountains that have so inspired geographers and artists alike are,
surprisingly, just a small fraction of global topography.
Earth’s rigid lithosphere is broken into seven major and a number of minor
tectonic plates that move independently from one another, creating mountain belts and
volcanic arcs where plates converge and great rift zones where they spread apart (Figure
1.2). Many major surface features of the planet, including mountain ranges, earthquakeprone fault zones and volcanoes, occur along tectonic boundaries, but plate boundaries do
not necessarily coincide with the edges of the continents. Tectonic setting generally
determines the distribution of different rock types with different degrees of resistance to
erosion on Earth’s surface. For example, sedimentary rocks found in ancient depositional
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basins that have been uplifted and exposed are susceptible to rapid erosion, whereas highgrade metamorphic rocks at the cores of deeply exhumed ancient mountain belts wear
down much more slowly. The global distribution of rock types and rates and styles of
crustal uplift, deformation, and extension reflect the history and patterns of tectonic
forcing that set the stage for topographic evolution and landscape development.
Tectonics
Tectonic plates move independently of one another as the lithosphere is rafted
along by thermal convection of Earth’s deep mantle. Plate margins are classified as
divergent, convergent, or transform. Divergent margins are those where two plates
spread apart. Convergent margins are those where two plates move toward and collide
into one another. Transform margins are those where two plates slide laterally past one
another, like along California’s earthquake-prone San Andreas Fault Zone (Figure 1.4).
Mid-ocean ridges are divergent plate boundaries where mantle upwelling at
seafloor spreading centers creates new oceanic lithosphere that is pushed aside and
moves away from the ridge as new material is extruded. The new ocean crust sinks as it
cools and moves away from the ridge, and this basaltic material forms the floors of
Earth’s oceans. Subduction zones are convergent plate boundaries where denser oceanic
lithosphere sinks beneath a lighter continental plate or a younger, more buoyant oceanic
plate. (The plate boundary on the Pacific side of the Andes in South America and the
trench seaward of Japanese islands are examples of subduction zones.) In subduction
zones, the down-going slab of dense oceanic crust pulls along the trailing crust and
provides another driving force for tectonics. The combination of spreading centers and
subduction zones on opposite sides of lithospheric plates defines the top halves of the
deep convective cells that drive plate tectonics. Less dense continents and islands do not
subduct and are along for the ride as plates move apart, collide with, dive beneath, or
slide past one another.
Continental margins are called active margins where they coincide with plate
boundaries and passive margins where there is no relative motion between the continent
and the seafloor, as for example along the east coast of North America. Great mountain
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belts, like the Himalaya, form in continental collision zones where two continents made
of low-density crustal rocks that cannot subduct are squeezed between converging plates.
In the case of the Himalaya, the Indian sub-continent has been ramming into Asia for the
past 40-50 million years. The collision has piled up crust to form the Tibetan Plateau,
much like how sand piles up in front of an advancing bulldozer blade. Continental
collision zones build high mountains because crustal thickening gradually elevates the
surface while building a deep root. Mountain belts also form above subduction zones
where material scraped off the down-going slab piles up and volcanism results from
partial melting of the slab at depth. Tectonically active mountain belts, like the
Himalaya, the Andes, and the Cascades in the Pacific Northwest, are found along
convergent margins (collision zones and subduction zones). Others, like the
Appalachians and the Urals, record ancient continental collisions at plate boundaries that
are no longer tectonically active. Where sustained for long enough crustal thickening in
excess of erosion can exceed the height that the mechanical strength of the crust can
support, leading to development of high plateaus, like Tibet and the Altiplano, as further
crustal convergence simply expands the lateral extent of the region of high terrain.
Continents are composed of mountain belts called orogens (sometimes referred to
as orogenic belts), ancient, tectonically stable and generally low-relief cratons, and
extensional rift zones (Photo 1.2). Mountain belts typically consist of sedimentary and
crystalline rocks that are highly deformed and erode rapidly, at rates up to several
kilometers per million years (several millimeters per year). In contrast, cratonic
landscapes, like the interior of Australia, erode very slowly at rates much less than a
meter per million years. Slowly eroding cratonic terrain is typically composed of ancient
crystalline rocks that have been tectonically stable for tens or even hundreds of millions
of years. Rift zones, however, like humanity's ancestral home in East Africa are places
where active divergent plate boundaries extend into continents, tearing landmasses apart,
raising steep rift flanks, and emplacing relatively young volcanic rocks. Continental
shelves are submerged alluvial plains at the edges of the continents that formed during
sea level lowstands. These areas were repeatedly exposed to sub-aerial erosion as cycles
of glaciation periodically locked enough ice up on the continents to significantly lower
sea level. Continental shelves extend beyond present day shorelines, and they vary from
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narrow features a few kilometers wide on tectonically active margins to broad areas of
relatively shallow marine inundation that extending hundreds of kilometers off shore on
passive margins.
Within tectonic plates, upwelling plumes of hot rock from deep in the mantle,
called hot spots, dramatically influence topography because they produce prodigious and
intense volcanism. In oceanic settings, hot spots build chains of islands and seamounts
like the Hawaii-Emperor chain. In continental settings, they create volcanic calderas, like
the Yellowstone caldera in Wyoming, and Long Valley in eastern California. Because
hot spots are rooted in the mantle below the lithosphere, they remain stationary as plates
move over them, and they result in topographic features that track the direction and rate
of plate movement. For example, in the linear volcanic chain of the Hawaiian Islands the
younger islands lie progressively south-eastward of older islands, a trend that translates
into systematic differences in soil properties (notably soil fertility) and degree of
topographic dissection (valley incision) with increasing island age.
Lithology and Structure
Lithology (rock type) and geologic structures like sedimentary layering, faults,
and folds greatly influence erosional resistance, and affect the style of weathering and the
soil it produces, the durability of weathered material, and the strength of hillslopes.
Different rock types vary greatly in their chemical composition, texture, and material
strength. The stair-stepped walls of the Grand Canyon, in which geological formations
determine slope steepness — hard units holding up steep cliffs and weak units forming
topographic ledges — is an example of how lithology affects topography (Photo 1.3).
Rock structure also plays a key role in determining erosion resistance, because the
degree to which rocks have been tectonically fractured, sheared, or deformed influences
their material strength. Sedimentary rocks are generally stratified, and have structures
like bedding planes that create material discontinuities and planes of weakness, while
crystalline rocks, like granite, are typically more massive and stronger. The degree of
consolidation and the amount and type of interstitial cement also affect the strength and
geomorphic expression of sedimentary rocks. Preferentially oriented cleavage planes and
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fabrics in metamorphic rocks often dominate their geomorphic expression. Geologic
structures that expose rocks of variable erosion resistance can generate distinctive
topographic signatures that allow geologists to recognize features like folds, faults, and
fracture patterns solely from their geomorphic expression (Photo 1.4).
Different rock types weather at different rates and produce weathering products
with variable material strengths and other properties that influence landform
development. For example, variable weathering and erosion often lead to hill and valley
terrain with high-standing topography capped by hard, erosion-resistant rock (like quartz
sandstone) and areas of subdued topography that are underlain by relatively weak rock
that is more susceptible to weathering and erosion (such as siltstone and shale). Rocks of
a particular lithology may, however, behave differently in different climates. Easily
weathered rocks that hold vertical cliffs where they are exposed in arid landscapes may
only support gentle slopes in humid or tropical environments with more vigorous
weathering. The patterns of rock types resulting from the geologic history of a region
provide the raw material from which regional topography develops.
HYDROSPHERE
The distribution and movement of water over Earth’s surface greatly influences
landscape evolution, and the hydrosphere is defined as that part of Earth’s crust and
atmosphere over and through which water circulates. How water moves down slopes and
through the subsurface to collect in streams affects both weathering and erosional
processes. Overland flow in which rainfall simply runs off across the land surface after
striking the ground is rare in humid and temperate landscapes because of uptake by
plants, depressions that concentrate flow, and absorptive soils. In these environments,
most water sinks into the ground (infiltrates), and shallow subsurface flow is much more
common as water moves through soils and shallow permeable rock on the way to rivers
and streams. Subsurface flow also influences rock weathering and soil development.
Overland flow is, however, often an important runoff mechanism in arid regions and
disturbed areas where the soil has been compacted, degraded, or paved over. The
dynamics of glaciers and seasonally frozen ground can dominate geomorphological
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processes in alpine and high latitude regions. The amount of water, its form (rain, snow
or ice), and how it moves through and across the landscape are major controls on
weathering, erosion, and landscape evolution.
Climate and Climate Zones
Climate describes long-term spatial and temporal patterns of precipitation and
temperature on Earth’s surface. As such, it characterizes the average and variability of
the daily phenomena of weather. Solar radiation provides the energy that shapes Earth’s
climate; it evaporates water from the oceans into the atmosphere, drives atmospheric
circulation, and indirectly produces wind and storms. About half of the energy our planet
receives from the sun is reflected back into space by clouds, particulate matter in the
atmosphere, and the ground surface. The amount of incoming solar energy the land
surface absorbs, called insolation, varies with latitude, producing a temperature gradient
between the poles and equator. The resulting atmospheric and oceanic circulation
produces substantial pole-ward transfer of heat that delivers warm air and water to higher
latitudes. Because Earth’s total heat budget is relatively constant, well-defined
temperature and precipitation patterns impart different, predictable climates to different
regions. Over time, global and regional climates shift in response to changes in solar
insolation, including those that arise from eccentricities in Earth’s orbit and from
fluctuations in the concentrations of heat-trapping greenhouse gasses, and have done so
many times through Earth history. Of particular concern today are changes in greenhouse
gases — including the CO2 released to the atmosphere from burning fossil fuels — that
can change Earth's climate by altering the amount of solar radiation trapped by the
atmosphere. Past climates and changes in climate have greatly influenced landforms and
ecosystems in many parts of the world. Geomorphological processes respond to the
direct effects of climate change like changes in temperature and precipitation amount or
style (as in glaciation), as well as to indirect responses like sea level fluctuations or shifts
in ecological communities (such as a shift from grassland to forest).
Atmospheric circulation on our rotating planet gives rise to orderly circulation of
rising and falling air masses, and easterly versus westerly winds (Figure 1.5). These
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systematic latitudinal variations in temperature, precipitation, and wind create a pole-toequator sequence of climate zones that have distinctive ecological and geomorphological
characteristics. Climate patterns in the northern hemisphere are generally mirrored in the
southern hemisphere.
Latitudinal temperature differences give rise to three great convective cells of
atmospheric circulation. In the equatorial cell, called the Hadley cell, moisture-laden air
rises in the zone of high surface temperatures at the equator. As the air ascends, it loses
its capacity to hold moisture, and creates a zone of high precipitation in the tropical
equatorial zone. Atmospheric circulation in this zone is dominated by the equatorial
easterlies (trade winds) that consistently blow east to west. Between latitudes of about
30° and 35°, precipitation drops to a low beneath sinking, hot dry air masses in the
descending limb of the Hadley cell. Earth’s great deserts, like the Sahara, the Arabian
Desert and the Kalahari, are concentrated in this high-pressure sub-tropical belt. North of
the Hadley zone (or to the south in the Southern Hemisphere), the prevailing westerlies
that consistently blow west to east give rise to a temperate-zone convection cell called the
Ferrel cell, in which rising air masses produce another belt of high rainfall between 40°
and 60° latitude. In polar regions, the descending air of the third, or Polar cell delivers
little moisture, resulting in cold deserts at high-latitudes. These broad latitudinal patterns
not only shape regional climates but they influence individual weather events as well.
The trajectory of storm paths in the northern hemisphere temperate zone tracks the
unstable, constantly shifting jet stream at the boundary between the temperate and polar
cells.
Temperature and precipitation also vary with elevation. Within a mountain belt,
the local gradient in temperature and precipitation with elevation can be as great as the
variation between latitudinal climate zones. Differences in temperature and precipitation
define distinct climate zones that greatly influence the ecosystems and suites of
geomorphic processes in landscapes at different latitudes or elevations (Figure 1.6).
Glacial and periglacial processes are the main geomorphic influences in cold, highlatitude and high-elevation regions, rainfall predominates in temperate and tropical
regions, and wind plays an important role in shaping arid regions. Such differences
translate into distinct landforms characteristic of different climate zones (Photo 1.5).
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The distribution and orientation of mountains relative to prevailing winds and
moisture sources also affect rates of geomorphological processes. As moisture-laden
winds approach high mountains, air masses flow up and over the range. As wet air
ascends the windward slopes of a range, it cools, loses its ability to hold moisture, and
generates precipitation that nourishes lush vegetation on the upwind side of the
mountains. By the time an air mass reaches the downwind side of a high mountain range,
it has lost substantial moisture. Warming as it descends, this low-humidity air continues
on to form arid regions and deserts downwind of major mountain ranges. The orographic
effect of topography wringing moisture out of the atmosphere results in the development
of so-called rain shadows on the leeward sides of mountain ranges (and substantial
oceanic islands like Hawaii) that tend to be drier than their windward sides. Continental
interiors are also drier than coastal areas. Such differences profoundly affect the local
climate, ecological communities, and erosion rates that influence the development of
topography.
Hydrologic cycle
The movement and exchange of water between the oceans, land, and atmosphere
is known as the hydrologic cycle. Water evaporated from the ocean surface rises
together with warm air to become clouds that move with winds that blow air masses over
continents. Precipitation that falls as rain or snow either infiltrates into the ground, runs
off over the ground surface to join bodies of surface water, evaporates, or is transpired by
plants back to the atmosphere. Whether it is slow-moving groundwater or relatively
quickly flowing surface and near-surface runoff, water eventually flows into stream
systems that ultimately discharge into the sea. The distillation of fresh water from the
oceans, its delivery to the continents, and subsequent return to the oceans drives key
geomorphological processes (Figure 1-7). Most of the water on Earth (97%) is held in
the oceans. About 2 percent is stored as ice in glaciers and polar ice sheets and about 0.6
percent is stored as groundwater. Freshwater rivers and lakes account for less than one
half of one percent of the water on Earth. Humanity depends on the continuous
movement of water through the hydrologic cycle to maintain the freshwater supplies that
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sustain us, irrigate our crops, and support the terrestrial ecosystems upon which we
depend.
BIOSPHERE
The biosphere defines the realm of life, within, upon, and above Earth’s crust, and
includes organisms from invisible, short-lived soil microbes that inhabit geothermal vents
beneath the sea floor, to gigantic ancient redwoods that stretch skyward from mountain
flanks. Living systems depend on their environments and, in turn, influence the soils,
rocks, air and water in which they live. This interdependence is well illustrated by the
complex ecosystems that exist in the soils and fractured and weathered rock of the
critical zone between the abiotic, sterile geology of our planet’s interior and the biology
that covers much of its surface. The dynamics of earth surface processes control the
frequency and size of environmental disturbances like floods and landslides. Disturbance
processes may act to maintain stable ecological systems when they predictably recur in
the same place, such as how regular periodic inundation of floodplains leads to
characteristic riparian vegetation along river corridors. Conversely, disturbances that are
unpredictable and do not occur in the same place favor the evolution of non-equilibrium
life histories such as those for weedy plants that specialize in colonizing freshly disturbed
land. Understanding the relationships between ecological communities and the
landscapes they inhabit informs our understanding of both.
Geographical Distribution of Ecosystems
The global distribution of plant communities generally tracks global latitudinal
climate zones and the elevation-dependent patterns of temperature and precipitation in
alpine topography. Five broadly defined vegetation zones — semi-arid grasslands,
temperate forests, tropical forests, arid deserts, and polar regions — characterize the
global distribution of plant communities. Each zone has distinctive ground surface
coverage, reinforcement, soil types, and weathering properties that result from its plant
communities.
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Plants in grassland and forest zones not only generate different types of soils, but
their root systems reinforce soils differently so the landscapes differ in their resilience to
environmental disturbance as well. Grasslands generally have more biomass below
ground than above ground, most of it in roots. Grasslands are particularly vulnerable to
erosion following overgrazing and plowing. Temperate forests can be deciduous or
coniferous. In temperate forests, extensive root networks form interlocking webs that
mirror the extent of the forest canopy and significantly reinforce hillside soils.
Geomorphically important effects of forest type include the depth and strength of root
penetration, and the shape of fallen trees that enter rivers where the difference between a
long pole shape typical of conifers and the branching structure typical of deciduous trees
influences the stability and transportability of wood. Tropical forests typically have little
below ground organic matter and extensively weathered soils. As plant nutrients are held
in the plants themselves, it can be hard to re-establish native forests after forest clearing.
In contrast to temperate and tropical regions where vegetation can have a major
impact on the type, frequency, and intensity of geomorphological processes, vegetation
plays a relatively minor geomorphological role in arid and polar landscapes. However,
even in desert landscapes where plant communities have little biomass or plant cover, the
presence or absence of even a little ground cover or a thin web of roots can greatly affect
soil development. And frozen soils that support small trees and tundra vegetation can
accumulate and store lots of soil carbon due to slow breakdown. The potential for
warming tundra soils to release carbon into the atmosphere is one of the great concerns
surrounding potential feedbacks between climate change and ecosystem response.
The distribution of animal communities also generally tracks climate zones.
Penguins don't live in deserts, and camels don’t live on glaciers. Although most animals
only marginally influence the landscapes they inhabit, a few types of animals have a
substantial geomorphological influence. For example, mass spawning salmon move up
to half the gravel transported by some rivers in the Pacific Northwest. Overgrazing by
domestic animals accelerates soil erosion and can trigger gully development. Burrowing
animals and mound building ants and termites can displace and mix tremendous amounts
of soil and weathered rock. Charles Darwin calculated that over the course of centuries
worms steadily plowed the hillside soils of England.
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Plants and animals influence geomorphological processes directly, as when
burrowing activity and roots mechanically pry rocks apart (Photo 1.6), and indirectly, as
when plants protect soils from erosion during precipitation events, and roots
mechanically reinforce slope-forming materials. Plants also are central to the chemical
transformations that accompany the breakdown of rock-forming minerals into clay
minerals that hold nutrients essential to soil fertility. Plant communities thus shape how a
landscape weathers, erodes, and supports life. The size of an organism is not necessarily
related to its importance or impact. Soil bacteria, for example, are essential chemical
weathering agents in many environments.
Humans
Humans are today the most widely distributed species on the planet, and we move
enough rock and soil to count among the primary geomorphic forces shaping Earth's
modern surface. Coal and mineral mining operations move whole mountains and
excavate great pits, farmers’ plows push soil gradually but persistently downhill, and
construction crews cut or fill the land to facilitate building or to suit our aesthetic whims
(Photo 1.7). Human activities further influence geomorphological processes through the
indirect effects of our resource management and land use practices. In manipulating our
world, we alter hydrological processes by changing surface runoff, stream flow, and
flood flows. Clearing stabilizing vegetation and changing water fluxes in the landscape
affects slope stability, and increases erosion rates, and construction of dams and coastal
jetties interupts the transport and storage of sediment. Such changes often have
unintended consequences far downstream, as when upriver dam construction starves
beaches and deltas of sand and mud by trapping the sediment that formerly nourished
coastal environments. Learning to recognize and understand such connections is central
to applied geomorphology, whether to aid in the design resilient communities, develop
more sustainable land use practices, or construct measures to protect critical
infrastructure.
Through history human activities have resulted in changes to a wide range of
local and landscape-scale geomorphological processes that we are still learning to
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recognize and fully appreciate. For example, we are just coming to understand the
profound impact of Bronze Age forest clearing on European rivers. Further dramatic
changes in regional and global land cover are projected in the coming century. The
influence of human activity is already great enough on a global scale that geologists have
proposed we are entering a new era of geologic time that they call the Anthropocene (the
human era). Over the next century, changes in the global climate are predicted to cause
increasingly variable weather, more frequent hurricanes, rising sea levels, and a host of
related regional impacts, like the loss of winter snow pack in the Pacific Northwest.
Predicting the ways that landscapes respond to such changes will be central to planning
societal adaptation or mitigation efforts.
LANDSCAPES
Landscapes are suites of contiguous landforms that share a common genesis,
location, and history. The study of landscapes involves investigations over a tremendous
range of spatial and temporal scales, from the mobilization of a single grain of sand from
a river bed over a few seconds, to the rise of the Himalaya and growth of the Tibetan
Plateau over millions of years (Figure 1-8). Consequently, the approach, methods, and
scale of analysis involved in any particular study need to be tailored to the questions the
geomorphologist seeks to address. Landscapes can be divided into distinct units that can
be studied over discrete periods of time, but it is the relationships between processes and
landforms that are truly fundamental in our modern approach to geomorphology. In
seeking to understand landscape evolution it is important to appropriately match
measurement and understanding of geomorphological processes to the spatial and
temporal scales over which relevant processes act.
Process and Form
Landscape forms and surface processes are inseparably linked. Stream flow,
slope failure, flowing glaciers, and blowing wind act to shape landscapes. At the same
time, topography itself determines the style and rate of geomorphological processes. The
shapes and orientations of larger-scale landforms generally control the rates and
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distributions of the small-scale erosional and depositional processes that determine how
landforms evolve over time. The fundamental interdependence of process and response
means that a geomorphologist can often read form to infer process, as in determining
dominant wind directions from the shape and orientation of sand dunes. But in order to
understand landforms and predict landscape response, we must understand the processes
that form them. Consequently, the relationship between process and form lies at the heart
of geomorphology.
The geological and environmental history of a region can leave a lasting signature
on landforms and on the processes operating upon them. The physiographic signature of
long-vanished glaciers still dominates the topography of alpine regions around the world.
It should not be surprising then that process geomorphology and historical
geomorphology are complementary disciplines. Although many geomorphologists
specialize in one or the other approach, no geomorphologist can afford to ignore either.
Proposing testable hypotheses and interpreting field, laboratory, and simulation data in
the evaluation of those hypotheses requires consideration of processes over time because
of the fundamental linkages between landscape history, process, and form.
Landscape dynamics and evolution result from the interaction of multiple systems
that can be studied and understood in their own right. The topography of a mountain belt
reflects the interaction of tectonic processes that raise rocks above sea level with the
hydrologic and runoff processes that govern how erosion acts to shape slopes and incise
valleys. Considered broadly, the interaction of coupled tectonic, climatic, and erosional
systems control patterns of uplift, erosion, and sedimentation as they change over
geologic time. At finer scales, many geomorphological processes reflect the interaction
of ecological and hydrological systems, as when the binding effect of tree roots helps to
stabilize soils on landslide-prone slopes, or when log jams create dams that divert stream
channels to new courses across their floodplains. Understanding landscapes often
requires an appreciation of how systems interact in different regional contexts and
environmental settings.
Spatial Scales
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At global and continental scales, geomorphologists study major physiographic
features like mountain belts, depositional basins, and great river systems like the
Mississippi, Amazon, or Nile. At these scales, broad patterns in global climate and plate
tectonics influence patterns of erosion and deposition that, in turn, influence the size and
extent of mountains, plateaus, lowlands, coastal plains, and river basins. Such patterns
also affect the distribution of soil types and the relative importance of glacial, fluvial,
eolian (wind-driven), and coastal processes in shaping topography.
At a regional scale, distinct physiographic provinces reflect areas with broadly
similar history, landforms, and landscape dynamics. Examples of such provinces include
mountain belts like the Sierra Nevada or Appalachians as well as features like the Great
Plains, California’s Central Valley, and the Colorado Plateau in the American Southwest
(Figure 1-9). Physiographic provinces are areas in which similar suites of
geomorphological processes govern landscape formation and dynamics, and thus where
one finds similar suites of landforms.
A drainage basin is the land surface area drained by a given stream. Small
streams come together to form larger rivers, so landscapes are naturally organized into
smaller drainage basins nested within larger drainage basins. Drainage basins range in
size from a headwater catchment that collects water from a single mountain hillside to the
Amazon River basin that drains more than half of South America. Drainage divides are
topographic ridges that separate drainage basins. Because streamflow is the predominant
agent of erosion and sediment transport on land, drainage basins are the logical unit for
analysis of many geomorphological processes.
At the scale of individual valley segments, the tectonic or climatic setting of a
region often defines areas where different types of processes and/or histories have led to
development of distinct landforms and dynamics. The difference between U-shaped
valleys carved by glaciers valleys and V-shaped valleys cut by streams is a classic
example. Systematic downstream changes in stream valley morphology, from erosional
headwater streams with narrow valleys that are confined between bedrock walls, to broad
unconfined valleys of depositional lowlands. Distinct suites of valley segment types are
diagnostic of specific physiographic provinces, and their character and distribution both
18
vary regionally and generally reflect the history and processes of landscape evolution and
shape ecosystem dynamics.
At finer spatial scales, landscapes can be divided into distinct hillslopes, hollows,
channels, floodplains, and estuaries. Hillslopes (including hilltops) are the undissected
uplands between valleys. Hollows are unchanneled valleys that typically occur at the
head of channels in soil-mantled terrain. Channels are zones of concentrated flow and
sediment transport within well-defined banks, and floodplains are the flat valley bottoms
along river valleys that are inundated during times of high discharge under the present
climate. Estuaries are locations where streams enter coastal waters to arrive at their
ultimate destination — sea level. Each of these individual landscape units exhibits a
variety of specific landforms that are discussed in later chapters.
Temporal Scales
The scale of observations in both time and space strongly influence geomorphic
interpretations. Matching the scale of observation to the scale of the question you seek to
answer is critical for gathering meaningful data. It’s easy to understand the importance
of spatial scale. Measuring erosion on a single hillside in Kenya won’t tell you much
about the erosion rate of the African continent. It is more difficult to understand the
influence of time on geomorphic data, observations, and interpretations. Consider how
the first estimates of continental erosion rates, which were made by measuring the
concentration of suspended sediment exported by rivers over a few years, turned out to be
wrong. While this is a reasonable approach if the measurements capture the variability of
the system over time, in this case they did not. With only a few years of data, sediment
yield results are likely to be biased by the short period of observation because most
streams systems experience rare but massive floods that periodically transport immense
amounts of sediment. How would one know that the few years in which the sampling
occurred are representative of a meaningful, long-term average? The issue of how to
integrate the influence of large and small events in shaping larger landforms remains
central to modern process geomorphology.
19
Topography evolves over periods of time that range from the millions of years
that are required to erode away mountain belts, to the few seconds, minutes, or days it
takes for a landslide, flood, or earthquake-driven fault displacement to disrupt the land
surface. Climate cycles influence topography over millennia as glaciers advance, retreat,
and scour out alpine valleys. Likewise, river profiles adjust to the sea level changes that
accompany glaciations. Landscape responses to large-scale disturbances like hurricanes
and volcanic eruptions are often evident for centuries, and it can take decades for
landslide scars to revegetate and river channels to process the sediment shed from slopes
during large storms. River flow exhibits annual and seasonal variability that controls the
timing of sediment movement and structure of stream ecosystems. Because of the
disproportionate influence of infrequent extreme events like storms, landslides, and
floods, rates of processes measured over short time spans may not adequately
characterize average rates over longer time scales. Not surprisingly, geomorphologists
deal with a wide variety of measurements over different time scales, from rates directly
measured in the field, to indirect measurements of long term erosion rates inferred from
isotopic analyses, and erosion rates constrained by sedimentary volumes preserved in
depositional basins. The key, of course, is to employ analyses relevant to the time scale
of interest.
UNIFYING CONCEPTS
There are a number of core scientific concepts that are central to contemporary
geomorphology and guide our understanding and analyses of Earth’s dynamic surface.
Among these are the conservation of matter and energy, gravity-driven routing of
material through drainage basins, force balances and thresholds in natural systems, the
idea equilibrium and steady state balance of physical processes, and the relationship
between the magnitude and frequency of geomorphic events. These concepts are
fundamental to the discipline of geomorphology, and you will encounter them throughout
this book.
Landscapes are, in essence, physical systems that are dominated by the energydriven movement of matter through space. All geomorphic systems operate under the
20
constraint that energy is needed to move material from one place to another, and that
matter and energy are are neither created nor destroyed. Matter (material that has
physical mass) and/or energy is, however, almost always added or removed from
particular geomorphic systems, because the Earth system and its component subsystems
are interrelated open systems that trade matter and energy. Because gravity pulls water
and sediment downhill, the routes moving material takes from sources in eroding uplands
to sinks in depositional lowlands provide a common framework within which to consider
the action of geomorphological processes across landscapes. The balance between
driving and resisting forces allows us to quantify the ways that previously stable features
may change state or become destabilized. The frequency with which events of a given
magnitude occur is as important as their magnitude in considering their geomorphic
significance. Together these unifying concepts help structure a general, sytematic
approach to studying how earth surface processes interact to shape landscapes.
Conservation of Matter and Energy
The idea that matter and energy are neither created nor destroyed makes intuitive
sense. Rocks disaggregate as they weather, but they do not dematerialize. Eroded
material is transformed by chemical reactions or physical abrasion as it moves but it
always ends up somewhere else.
All landforms and geomorphic systems are open systems in which energy and
mass are continually removed or added. For example, a delta system at the mouth of a
river grows because of input of sediment from upstream sources and erodes away when
gravity and ocean currents remove more sediment than the river supplies. Similarly, a
mountain range erodes down to gentle hills over geologic time once tectonic uplift ceases
and erosion keeps removing mass and potential energy from the system. The concept of
tracking the flow of mass and energy through geomorphic systems gives us a powerful
framework for understanding earth surface processes.
In geomorphic systems, a simple yet very useful concept is how the difference
between inputs (I) and outputs (O) equals the change in storage (S):
I - O = S
(1-1),
21
where the Greek symbol delta, stands for change. The idea that input minus output
equals change in storage is simple and familiar. If you continually deposit money in the
bank, your balance increases. If all you do is spend, you deplete your savings. Do this
for long enough and you eventually run out of money. The same concept holds for soils,
rocks, and sediment. A mountain range eroding faster than tectonics raises it loses
elevation. A river that receives more sediment than it can transport river fills in and
aggrades its valley bottom.
Energy is likewise neither created nor destroyed, but only transformed. Potential
energy, the energy of position like that of a boulder perched high on a cliff, may be
converted into kinetic energy or heat. Imagine that same boulder rolling off the cliff and
crashing to the ground below. Its initial potential energy is transformed into kinetic
energy as it falls and picks up speed (with a small loss to heat by friction in the air).
When the boulder strikes the ground, the kinetic energy gained in the fall is transformed
to heat, sound, and kinetic energy that is imparted to anything it dislodges.
Topography itself sets the erosive potential of the primary forces acting to shape
landscapes, giving rise to the fundamental link between process and form. The potential
energy of rain and snowfall that is delivered to the land surface during precipitation
provides kinetic energy that erodes sediment and shapes the land. As water sinks into,
collects upon, and runs off over the land surface, its potential energy is converted to
kinetic energy in the form of flowing water or ice and dissipates through friction and the
transport of sediment. From there, water flows downhill under the influence of gravity,
and provides the energy for rivers and streams to maintain channels, transport sediment,
and incise valleys. The frequency and magnitude of precipitation across a landscape —
how much water falls where — determines the potential energy that is converted to
kinetic energy as runoff flows downhill, eroding and transporting sediment through
channel networks. Most of the kinetic energy of the flow is dissipated by friction and
turbulence generated as water moves across the channel bed and banks, leaving just a
fraction to do the geomorphic work of transporting sediment and eroding bedrock.
Material Routing
22
Material eroded from the headwaters of a drainage basin eventually makes its way
down through the river network, and sets the sediment supply for downstream channels.
Similarly, runoff generated in the uplands of a given basin determines the discharge
volume downstream. The one-way flow of water down river networks (and of ice down
glacial valley systems) is the predominant agent of erosion on the continents, so drainage
basins generally provide a logical framework for understanding landscape evolution in
many regions. Other forces govern coastal processes, the flow of continental ice sheets,
and eolian processes, but in general the downstream connections between different parts
of a landscape influence landscape dynamics, and different parts of a drainage basin can
be best understood in relation to the other parts of the basin.
Tracking material from upland sources to lowland depositional sinks is
particularly useful for considering how geomorphological processes interact to form and
shape landscapes. Headwater areas are generally steep, erosional regions where rocks
weather and break down into transportable material. Sediment mobilized from upland
slopes is delivered to streams and rivers and then transported to lowland depositional
environments, to the coast and ultimately the ocean. Drainage basins thus consist of
sediment source areas at their headwaters, central transport zones through which eroded
material moves by multiple episodes of deposition and erosion, and depositional zones
that serve as long-term sediment sinks (Figure 1.10). Because they are zones of erosion,
steep headwaters are often not preserved in the geologic record, and we rely mainly on
the geologic record of depositional environments where material stripped from erosional
source areas was deposited near or below sea level to reconstruct the erosional history of
continental highlands. A source-to-sink framework allows considering how the erosion,
transport, deposition, and storage of sediment drive landscape evolution and dynamics.
Force Balances and Thresholds
Force balances are fundamental to our understanding of geomorphic processes,
particularly the balance between driving stresses imparted by gravity and resisting forces
offered by Earth materials. Force balance calculations help geomorphologists analyze a
diverse set of processes including, but not limited to, slope stability, the movement of
23
sediment along the bed of a river, and the flow of ice down glacial valleys. Generally,
the ratio between driving and resisting forces is determined using trigonometric functions
and vectors to resolve gravitational driving forces in the down-slope direction, and
constitutive equations are used to quantify how Earth materials like rock, soil, and ice
resist deformation or failure (Figure 1.11).
In considering the relationship between an applied force and the surface it acts
upon, it is useful to resolve the force into normal and shear components that respectively
act orthogonal to (normal) and parallel to (shear) the land surface. Gravity is the primary
driving force for most geomorphological processes, so we can use trigonometry to
resolve forces arising from the unit weight (the product of density, , and gravitational
acceleration, g) of a column of rock, soil, water, ice, or air of thickness z, on a surface,
like a hillside or river bed that slopes at an angle  into normal and shear components.
The normal force is given by •g•z•cos and acts to resist downhill movement and hold
material in place. The shear force is given by •g•z•sin and acts to impel material
downslope.
Geomorphic thresholds are physical or chemical conditions that, when reached,
dramatically alter the behavior of a system or disrupt its equilibrium enough to trigger a
change in state or a shift to a new range of average conditions (Figure 1-12). The
concept of geomorphic thresholds is important because many landscape processes and
landforms are prone to the influence of individual events. The idea is that a dynamic
geomorphic system remains stable until an event of sufficient strength tips the balance,
and crosses a threshold to another system state. For example, small and moderate floods
may rush down a stable bedrock channel for decades until, one day, a flood of sufficient
size exceeds a threshold and causes dramatic changes. On that day, floodwaters exert
enough shear stress to pluck a slab of rock from the bed, move it downstream, and
irreversibly alter the channel. Thresholds are important when considering a variety of
geomorphic environments and processes including mass movements, sediment transport,
and volcanic eruptions. A central implication of the concept of geomorphic thresholds is
that small changes can trigger large responses. Changes triggered by crossing
geomorphic thresholds sometimes also cause a cascade of responses referred to as a
24
complex response, in which different parts of a system reach the threshold conditions at
different times. This can lead to responses that may be out of phase in different portions
of the channel network, such as downstream channel reaches incising while upstream
reaches remain sediment choked.
Equilibrium and Steady State
The idea of balance, or equilibrium, between landforms and geomorphological
processes provides a useful conceptual framework and hypothesis through which to study
landscape evolution, as well as a reference frame for identifying and understanding nonequilibrium landforms and landscapes. Many models and calculations assume that a
landscape in equilibrium, a condition referred to as steady state, does not change over
time. Equilibrium is, however, often not static, but is rather a dynamic steady state, with
landscape parameters that vary over time around a stable mean value. Steady state and
equilibrium are strongly scale-dependent. The average slope of a mountain range, for
example, remains constant if erosion and rock uplift rates are equal over time, even if
individual erosional events dramatically change portions of the landscape in the short
term. The time scales over which topography equilibrates to changes in landscapeforming processes range from seasonal re-surfacing of gravel streambeds following
winter storms, to the tens of millions of years it can take to erode mountain ranges.
Most people may view landscapes as unchanging, but considered geologically
topography is quite dynamic and likely never reaches a perfect steady state because
material is constantly being entrained, transported, and deposited. Over centuries to
millennia, such subtle changes result in a dynamic equilibrium that maintains
topographic forms in an average sense even as individual slopes experience landslides,
coastal landforms shift with currents, tides and storms, and rivers migrate across their
floodplains. Over longer time scales, sometimes referred to as cyclic time, landforms
evolve in concert with tectonic and climatic changes. For example, once tectonically
driven rock uplift ceases, mountains slowly erode away and average slopes decline. The
response time of a landscape or landform to changes in driving or resisting forces varies
greatly depending on the type of change, the rates of landscape forming processes, and
25
the resistance of the landscape to a particular change. There are many ways to consider
and quantify geomorphic equilibrium, but all landscapes reflect the shifting balance
between driving and resisting forces, and the response time to changes in either.
Recurrence Intervals and Magnitude/Frequency Relationships
Most changes on Earth’s surface happen in response to discrete events that disrupt
the land surface, move mass, and change surface forms. Geomorphic disturbances come
in a wide range of scales, from massive volcanic eruptions and hurricanes with 200 km
hr-1 winds and storm surges several meters high, to a lone tree falling over, mixing the
soil held by its roots, or the failure of a single hillside (Photo 1.8). The timing of
periodic disturbances and the duration of geomorphic events both vary over many orders
of magnitude. Disturbances also affect the links between biotic and abiotic processes
because plants often rapidly recolonize areas left bare by catastrophic events like
landslides and floods.
Events of a certain magnitude often reoccur with a regular probability, and
geomorphic events, while randomly distributed in time, generally have a characteristic
recurrence interval, the average time between events of a similar magnitude. Take for
example, a flood that is just barely capable of overtopping the banks of a river, a level
termed bank-full flow. A flood of this magnitude occurs, on average, once every year or
two in most humid/temperate zone streams, while the recurrence interval of bank-full
flow may exceed 50 years in arid region channels. Many events that are important in
shaping landscapes, including earthquakes, floods, and hurricanes, have discrete
magnitude/frequency relationships that quantify the relationship between the size of an
event and the chance that it will occur in a given time period (Figure 1-13). It is
important to note that different environments can have different recurrence intervals for
the same process, a result of differing climate and tectonic setting.
APPLICATIONS
Understanding the relationship between geomorphological processes and
landforms has applications across all aspects of human endeavor, from how to
26
sustainably feed a growing population, to military planning, disaster preparedness, landuse planning, ecological restoration, and space exploration. The dynamic nature of
Earth's surface means that geomorphology has important ties to other disciplines — from
civil engineering, to agriculture, and ecology. It also means that fundamental
understanding of geomorphological processes lays the foundation for applied
geomorphology.
Insight into geomorphology plays a key role in natural hazard assessment,
prevention, and recovery. Our ability to recognize the potential for landslides, floods,
earthquakes, storms, and sinkholes is essential in efforts to minimize loss of life and
property destruction, to mitigate or repair damage, and to modify our behavior to
minimize future damage from these hazards. Geomorphology is an essential foundation
for rational land-use planning and landscape management, particularly for landscapescale natural resource management, like forestry. In particular, understanding how
natural processes shaped landforms and river systems is a key part of the foundation for
developing strategies for mitigating human impacts or designing ecological restoration
efforts.
The ability to read the landscape has proven invaluable for military planning for
millennia. History is replete with examples of how knowledge of physical geography led
to military triumph, and how ignorance or neglect led to disaster. Historians speculate
that the Delaware River was frozen when George Washington crossed his rag-tag army
across it in the dead of night to the great surprise of the Hessan garrison in Trenton, New
Jersey. During the Korean War, the U.S. Army was forced to begin what became a
massive retreat from its defensive perimeter around the Korean capital of Seoul in
January, 1951, when the Han River froze and the Chinese Army crossed as if it was not
there. Thick layers of dust hidden just below the rocky desert surface in Iran, Iraq, and
Afghanistan have plagued helicopters that were originally designed to maneuver and
deploy troops in the humid and muddy, but dust-free, jungles of Vietnam. Knowing what
to expect form the terrain — and how to use it — has proven invaluable time and again
through history.
27
Spectacular advances in planetary geomorphology have extended the study of
landscapes to other planets and moons, culminating most recently in directly interactive
robotic fieldwork on Mars with the successful landing of the Mars Rovers. Exciting
challenges in the exploration of other worlds will continue to be informed by analogs and
lessons learned here on our home planet. While much is known about the nature of
Earth's dynamic surface, landscapes around the world still harbor untold stories waiting
for our interpretation through general principles of landscape evolution and dynamics.
28
SELECTED REFERENCES AND FURTHER READING
Anderson, S. P., von Blanckenburg, F., and White, A. F., 2007, Physical and chemical
controls on critical zone form and function, Elements, v. 3, p. 315-319.
Bull, W. B., 1991, Geomorphic Responses to Climate Change, Oxford University Press,
New York and Oxford, 326p.
Butler, D. R., 1995, Zoogeomorphology: Animals as Geomorphic Agents, Cambridge
University Press, Cambridge, 231p.
Chorley, R. J. (editor), 1969, Water, Earth, and Man: A Synthesis of Hydrology,
Geomorphology and Socio-Economic Geography, Methuen & Co., London.
Chorley, R. J., Dunn, A. J., and Beckinsale, R. P., 1964, The History of the Study of
Landforms, or the Development of Geomorphology, Methuen & Co., and John
Wiley & Sons, Inc., Frome and London.
Coates, D. R., and Vitek, J. D., (editors), 1980, Thresholds in Geomorphology, George
Allen & Unwin, London, 498p.
Darwin, C., 1881, The Formation of Vegetable Mould, Through the Action of Worms,
With Observations on Their Habits, John Murray, London.
Davies, G. L., 1969, The Earth in Decay: A History of British Geomorphology 15781878, American Elsevier Publishing Company, New York.
Derbyshire, E. (editor), 1976, Geomorphology and Climate, John Wiley & Sons, London.
Dietrich, W. E., Wilson, C. J., Montgomery, D. R., McKean, J., and Bauer, R., 1992,
Erosion thresholds and land surface morphology, Geology, v. 20, p. 675-679.
Hack, J. T., 1960, Interpretation of erosional topography in humid temperate regions,
American Journal of Science, v. 258A, p. 80-97.
Hassan, M. A., Gottesfeld, A. S., Montgomery, D. R., Tunnicliffe, J. F., Clark, G.K.C.,
Wynn, G., Jones-Cox, H., Poirier, R., MacIsaac, E., Herunter, H., and Macdonald,
S. J., 2008, Salmon-driven sediment transport in mountain streams, Geophysical
Research Letters, v. 35, L04405, doi:10.1029/2007GL032997.
29
Hooke, R. L., 2000, On the history of humans as geomorphic agents, Geology, v. 28, p.
843-846.
Kirchner, J. W., Finkel, R. C., Riebe, C. S., Granger, D. E., Clayton, J. L., King, J. G.,
and Megahan, W. F., 2001, Mountain erosion over 10 yr, 10 k.y., and 10 m.y.
time scales, Geology, v. 29, p. 591-594
Molar, P., and England, P., 1990, Late Cenozoic uplift of mountain ranges and global
climate change: Chicken or egg?, Nature, v. 346, p. 29-34.
Montgomery, D. R., 2007, Soil erosion and agricultural sustainability, Proceedings of the
National Academy of Sciences, v. 104, p. 13,268-13,272.
Montgomery, D. R., Balco, G., and Willett, S. D., 2001, Climate, tectonics, and the
morphology of the Andes, Geology, v. 29, p. 579-582.
Reid, L. M., and Dunne, T., 1996, Rapid Evaluation of Sediment Budgets, Catena-Verlag,
Reiskirchen.
Summerfield, M. A. (editor), 2000, Geomorphology and Global Tectonics, John Wiley &
Sons, Chichester.
Syvitski, J.P.M., Vorosmarty, C.J., Kettner, A.J. and Green, P., 2005, Impact of humans
on the flux of terrestrial sediment to the global coastal ocean, Science, v. 308, p.
376-380.
Thomas, W. L., Jr (editor), 1956, Man's Role in Changing the Face of the Earth, The
University of Chicago Press, Chicago.
Thornes, J. B., and Brunsden, 1977, Geomorphology and Time, John Wiley & Sons, New
York.
Whipple, K. X., 2009, The influence of climate on the tectonic evolution of mountain
belts, Nature Geoscience, v. 2, p. 97-104.
Wilkinson, B. H., and McElroy, B. J., 2007, The impact of humans on continental erosion
and sedimentation, Geological Society of America Bulletin, v. 119, p. 140-156
Willett, S. D., and Brandon, M. T., 2002, On steady states in mountain belts, Geology, v.
30, p. 175-178.
30
Willett, S., Beaumont, C., and Fullsack, P., 1993, Mechanical model for the tectonics of
doubly vergent compressional orogens, Geology, v. 21, p. 371-374.
Wolman, M. G., and Miller, J. P., 1960, Magnitude and frequency of forces in
geomorphic processes, Journal of Geology, v. 68, p. 54-74.
31
B
A
C
E
D
F
Photo 1.1 Why geomorphology matters: A. The Snoqualmie River near Carnation,
Washington is flooded by a rain-on-snow event and now covers its floodplain. Photo by
P. Bierman. GEOMORPH0000000118. B. Looking upstream at a landslide that blocked
a stream channel in the Alaska Range, Alaska, creating a temporary lake. Photo by A.
Matmon. GEOMORPH0000000742.jpg. C. Contrasting bouldery matrix supported
debris flow deposit on the right and clast supported streamflow deposit on the left. Photo
by P. Bierman. GEOMORPH0000000223.jpg D. A house tumbles into the sea as the
permafrost below melts and the coast becomes more susceptible to erosion. Photo by B.
Jones, USGS, GEOMORPH0000002816.jpg. E. Perito Moreno Glacier in Patagonia,
Argentina calved ice as it melts in a warming climate. Photo by L. Galuzzi.
GEOMORPH0000001163. F. Bouldery Fault scarp at site of 1872 rupture on Owens
Valley fault zone, southern California. The land on the left moved up several meters
exposing the boulders. Photo by M. Miller. GEOMORPH0000001279
32
A
B
C
Photo 1.2 Continental landscapes. A. The Sella mountains in the Dolomite Range of ,
Italy are steep and shedding large cones of talus. Photo by M. Geilhausen.
GEOMORPH0000000857. B. The Dead Sea rift shoulder in Israel is steep, its
topography controlled by a rift-bounding fault. Photo by P. Bierman.
GEOMORPH0000000076.jpg. C. The Australian craton or shield is in large part nearly
flat-lying with little relief and exposing very weathered crystalline rocks. Photo by P.
Bierman, GEOMORPH0000000086.
33
Photo 1.3. The Grand Canyon of the Colorado River in Arizona. Here, rock type controls
topography with strong rocks such as well cemented limestones forming steep cliffs and
weak rocks, such as shale, holding lower slopes and being buried in debris. Photo by L.
Galuzzi, GEOMORPH0000001139.
Photo 1.4. This plunging anticlinal structure is clearly visible because more resistant beds
are outlined by weathering and erosion. Southwestern Montana. Photo by Marli Miller.
GEOMORPH0000001361.
34
A
B
C
D
E
Photo 1.5. Climate types dramatically influence geomorphic processes and the
appearance of landscapes. A. Largely devoid of vegetation, hillslopes in the Atacama
Desert of northern Chile are gullied in rare but intense rainfalls. Photo by T. Schildgen,
GEOMORPH0000000307. B. Arctic landscapes are often dominated by ice and
diminutive vegetation, stunted by the cold and short growing season such as here, in
Greenland near a recently drained glacial-margin lake. Photo by P. Bierman,
GEOMORPH0000000292. C. Face of an open pit mine showing how tropical landscapes
are dominated by extreme weathering. Here, in Brazil, the depth of weathering is shown
by iron oxidation in rock tens of meters below the surface. Photo by D. Montgomery.
GEOMORPH0000001020. D. In humid-temperate zones, where much of Earth’s
population lives, much of the landscape is tree-covered such as this view of the southern
Appalachians in West Virginia. Photo by D. Thompson. GEOMORPH0000001701. E.
Alpine landscapes are often steep, rocky and cold with little vegetation and the persistent
effects of glaciation such as here, at Convict Creek in the Sierra Nevada of California.
Photo by M. Miller. GEOMORPH0000001417.
35
Photo 1.6. Tree and root growth slowly works to wedge this rock apart in Ledyard,
Connecticut. Photo by D. Thompson, GEOMORPH0000002376.
Photo 1.7. Face of an open pit mine showing how tropical landscapes are dominated by
extreme weathering. Here, in Brazil, the depth of weathering is shown by iron oxidation
in rock tens of meters below the surface. Photo by D. Montgomery.
GEOMORPH0000001020.
36
Photo 1.8 Disturbances come in many different length scales. A. Blow down of a single
tree in the Adirondack mountains of New York State, disturbs several tens of square
meters. GEOMORPH0000000322.jpg. B. Blown down trees from the 1980 eruption
(lateral blast) of Mt. St. Helens volcano in Washington State. The blast leveled about 600
square kilometers of timber.GEOMORPH0000000553.jpg [HOW IS B A
GEOMORPHIC DISTURBANCE???]
37
FIGURE CAPTIONS
FIGURE 1.1 The surface of the Earth has two dominant elevations – the deep ocean
basins at an average depth of about 4000 m below sea-level and the continental
cratons where most of the land is less than a 1000 meters above sea-level. Much
of Earth’s exposed land is in the Northern Hemisphere.
FIGURE 1.2 The character of Earth’s dynamic surface reflects the forces acting upon it.
The movement of tectonic plates, the shaking of earthquakes, and the eruptions of
volcanoes all influence geomorphology, and are all related.
FIGURE 1.4 Each type of plate tectonic boundary (convergent, divergent and transform)
is associated with a particular suite of landforms allowing geomorphologists to
predict landscapes and active surface processes in different parts of the world.
FIGURE 1.5 Air and water circulate through Earth’s atmosphere and oceans – moving
heat around Earth in response to unequal input of energy from the sun over time
and over the surface of our planet.
FIGURE 1.6 Earth’s has distinctive and predictable climate zones determined by
precipitation and temperature. These zone shift over geologic time as climate
warms and cools. If atmospheric CO2 continues to increase rapidly as a result of
fossil fuel combustion, climate zones may once again shift, this time over just a
few human lifetimes.
FIGURE 1.7 Water moves through the Hydrosphere in vapor, liquid and solid forms.
FIGURE 1.8 Scale is critical to understanding the geomorphology of Earth’s surface.
Some processes happen on short timescales and over small distances while others
are imperceptibly slow and on the scale of continents.
FIGURE 1.9 Physiographic Provinces of the United States / Drainage Basins /
Landscape Units (hillslopes, hollows, channels, and floodplains).
FIGURE 1.10 It’s useful to understand Earth’s surface from the perspective of drainage
basins – units of the landscape in which mass can be accounted and conserved. In
38
general, sediment is sourced in the eroding uplands and deposited in lowland
sinks.
FIGURE 1.11 Force balances are a physical way of understanding Earth’s Surface. The
dynamic interaction between driving forces that tend to move objects and
resisting forces that tend to prevent movement can be used to explain a diverse
range of surface processes. [DEFINE / SHOW SHEAR AND NORMAL
FORCES].
FIGURE 1.12 Thresholds, equilibrium, steady state and response time.
FIGURE 1.13 At Earth’s surface, large scale or high intensity events are rare and small,
lower intensity events are common. Yet, large events may have dramatic effects
on the landscape.
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